Various electronic devices are available on the market that transform a digital or electronic image to appropriate density variations on an image carrier, in order to render the electronic image visible on the image carrier. Alternatively, the electronic image is converted to an image-wise distribution of ink repellent and ink accepting zones on a printing plate, for use in e.g. offset printing.
An electronic image is typically represented by a rectangular matrix of pixels, each having a pixel value. The location of each pixel within the matrix corresponds to a specific location on the image carrier. Each pixel value corresponds to an optical density required on the image carrier at the specific location.
In a binary system, two pixel values, e.g. 0 and 1, are sufficient, to represent a high density and a low density, which may be obtained by applying ink and no ink respectively, or toner and no toner, or generating locally dye or no dye, or by keeping and removing silver in a photographic process. In the production of printing plates, 0 may result in an ink repellent zone, where 1 results in an ink accepting zone.
In a continuous tone system, multiple density levels may be generated on the image carrier, with no perceptible quantisation to them. In order to achieve such fine quantisation, usually 256 different density levels are required, such that each pixel value may range from 0 to 255. In electrophotography usually a reduced number of density levels can be generated consistently, e.g. 16 levels, in which case the system is called a multilevel system, as opposed to a binary system or a continuous tone system.
As said before, each "zone" or "microdot" on the image carrier gets a density, corresponding to a pixel value from the electronic image. Such a zone is indicated by the term "microdot." A microdot is the smallest space on the image carrier that can get an optical density (or ink repellency) different from neighbouring locations. Usually microdots are represented by squares or rectangles within parallel and orthogonal grid lines. The spacing of the grid lines is indicative for the resolution of the output device.
For each microdot on the image carrier, one pixel value is required. In an output device, based on image generation by exposure to light, the microdots are usually illuminated sequentially one at a time by (one or more) scanning laser beams. Microdots may be illuminated one row at a time, as with emitting LED bars or through spatial light modulators like liquid crystal (LCD) shutters or digital mirror devices (DMD). Whereas in the case of a scanning laser beam, the exposure beam profile is essentially gaussian, the profile of the imaging elements with spatial modulators is determined by the projection optics and the intrinsic shape of the DMD mirrors in the case of DMD system; the shape of LCD electrodes in the case of LCD systems; typically the shape of the selfoc optics; and the emitting regions of the LED's in an LED bar.
A typical laser scanner example is the Agfa P3400 laser printer, marketed by Agfa-Gevaert N.V., which is a 400 dpi (dots per inch) printer. Each microdot has approximately a size of 62 .mu.m. The diameter of the circular spot is typically 88 .mu.m. This means that within a radius of 44 .mu.m the illuminance (W/m.sup.2) of the light beam is everywhere higher than 50% of the maximum illuminance. The illumination is usually nearly Gaussian distributed. This means that the illuminance is maximal in the centre of the microdot or in the centre of the circular spot, and decays as the distance from this centre increases. In some systems, an elliptical spot is preferred above a circular spot. Usually, the short axis of the ellipse is oriented along the fast scan direction of the laser beam, to compensate for the elongation of the illumination spot as the beam moves during the finite exposure times.
A typical LED exposure example is the Agfa P400 laser printer, marketed by Agfa-Gevaert N.V., which is equally a 400 dpi (dots per inch) printer and has an extension of the spot of typically 88 .mu.m.
The overlap of the illumination spots in these systems is designed as prescribed by an authoritative paper by H. Sonnenberg, titled "Laser-scanning parameters and latitudes in laser xerography," published by the Optical Society of America in Applied Optics, Volume 21, Number 10, 15 May 1982, pages 1745-1751. This paper gives an analysis of the width of lines in an image setting application with exposure by a scanning gaussian laser beam.
A problem with exposure systems, designed towards overlap along the diagonals of the rectangular microdot, is that the area exposed by single pixel illumination is larger than the rectangular microdot area as defined by the intersecting grid lines. This results in the fact that density contributions of a single isolated microdot as occurring in a halftone screen for the highlight portions of an image are larger than they should be.
In order to be able to compensate for this problem of tone shift in the highlights, screens of lower ruling or exposure systems of higher addressability are used, leading to lower quality or higher cost.
Another problem with electrophotographic systems, where the exposure system is designed towards overlap along the diagonals of the rectangular microdot, is associated with the use of contone, i.e. the use of different illumination energies at the microdot level, e.g. for purposes of multilevel halftoning. This problem becomes apparent when one considers the discharge characteristics of the photoconductor in the electrophotographic system as compared to the sensitometric response of graphical films.
Graphical films, used in conventional offset pre-press, invariably have a much steeper sensitometric response than organic photoconductors (OPC) used in electrophotographic printing systems, leading to a strongly nonlinear thresholding behaviour for the film, especially suited to binary offset printing with reasonable exposure latitude.
In an electrophotographic system, multilevel exposure at the microdot level is used to reduce tone gradation coarseness at a given screen ruling associated with the limited addressability. Exposure intensity at the pixel level is varied and the operation point on the discharge curve is chosen such as to have a nearly linear discharge behaviour as a function of exposure for most of the exposure range used.
Because of the smooth gradation response of the photosensitive medium, for which preferentially the conductivity varies when photons impinge on its surface, e.g. an organic photoconductor (OPC), an essentially uniform energy distribution within the microdot is required.